Secondary-emission prevention circuit for use with vidicon tubes and the like



"May 19, 1964 HQROWITZ 3,134,043

SECONDARY-EMISSION PREVENTION CIRCUIT FOR USE WITH VIDICQN TUBES AND THELIKE Filed Sept. 18, 1951 IV r . r1050 AMI? IN VEN TOR. l/zwms llaeowxrzATTORNEYS United States Patent ()fiice 3,134,043 SECONDARY-EMISSIONPREVENTION CIRCUIT FOR USE WITH VIDICON TUBES AND THE LIKE IrvingHorowitz, Eatontown, N.J., assignor to Blonder- Tongue Electronics,Newark, N.J., a corporation of New Jersey Filed Sept. 18, 1961, Ser. No.138,891 4 Claims. (Cl. 315-'10) The present invention relates tosecondary-emission prevention circuits, and more particularly, to suchcircuits as may be employed with the targets of vidicon tubes and thelike.

It has for some time been customary to connect the target of a vidicontube or the like through a high-series resistance (of value comparablewith that of the vidicon i.e. the target-to-ground resistance) to asource of potential for the purpose of providing automatic compensationfor different intensities of light striking the target. The voltage dropacross the series resistance will increase with increasing lightintensity striking the target, and this increase will result in tendingto reduce the actual target current toward its original value, therebyproviding substantially the same response irrespective of the intensityof the light impinging upon the target.

This type of automatic light compensation circuit, however, is subjectto certain practical difiiculties. When the vidicon is first turned on,its target will assume a high positive potential-as a result of thepresence of the compensating circuit, which may be sufiicient to causethe target to emit secondary electrons in substantial quantity. Nomatter how long the vidicon remains on, therefore, it will then becompletely incapable of operating in its intended fashion, since moreelectrons will be emitted by secondary emission than will be availableto scan the effects of light impinging upon the target. In suchinstances, attempts are made at the factory to lower the voltage that isapplied to the series resistance so that the initial vidicon targetvoltage, upon turn on, may be below the predetermined voltage at whichsubstantial secondaryemission takes place from the target.Unfortunately, however, this adjustment does not always work for manyvidicon targets in view of the differences in targets resulting frompresent-day manufacturing techniques, and many vidicons must accordinglybe rejected when they are actually really operative tubes, but are justnot adapted for operation in this type of circuit.

An objectof the present invention is to provide a new and improvedcircuit that enables such vidicons all to be employed in this type ofautomatic light compensation circuit, irrespective of variations intarget construction as a result of manufacturing techniques and thelike.

A further object is to provide a new and improved vidicon circuit.

Other and further objects will be explained hereinafter and will be moreparticularly explained in connection with the appended claims.

In summary, the objects of the present invention are attained byconnecting the target to a threshold-voltageoperated device of aparticular character that effectively connects the target to the vidiconcathode prior to the reaching of a condition where substantialsecondary-emission current would be emitted from the target.

The invention will now be described with reference to the accompanyingdrawing, the single figure of which is a schematic circuit diagramillustrating the invention in preferred form.

A vidicon of conventional type is schematically shown at 1 provided witha light-sensitive target 3 upon which electrons from an electron gun,generally designated by the reference numeral 5, may impinge, inresponse to accelerating voltage indicated as applied at the terminall-.

3,134,043 Patented May 19, 1964 While the invention is described inconnection with this vidicon type of tube, it will be evident that it isapplicable to other kinds of tubes of this character that become subjectto the same type of difiiculty. The target 3 is connected by a conductor7 through the before-mentioned series resistor R to a potentiometer Pthe slider S of which is adjusted to provide the compensating voltagebefore-discussed from a potential source labelled +ve, in order toenable automatic light-compensation effects to take place, as beforedescribed. The signal output from the target 3, resulting from scanningby the electron beam from the gun 5, is passed through capacitor C tothe videoamplifier stages, so labelled, as is well known.

As before discussed, when the voltage is first turned on, the targets 3of some tubes will assume the potential tapped off by the slider S, andthis potential may exceed the predetermined threshold voltage at whichthe target 3 becomes an effective or substantial secondary-emitter.Under such circumstances, the vidicon cannot work. To salvage the tube,one attempts to lower the position of the slider S in the hope offinding a voltage below the predetermined secondary-emission Voltage atwhich operation can be obtained. As before explained there are manyinstances where the particular type of target 3 is not adapted for thisparticular circuit and no such lower voltage can be found that stillprovides the desired automatic light compensation effect.

In accordance with the present invention, however, it is no longernecessary to reject vidicons i, that do not so Work. To the contrary, asupplementary circuit is connected by conductor 9 from the target 3 to athreshold voltage-operated device, preferably a device of the characterof a tube N having a pair of cold electrodes 2 and 4 disposed within aneon or similar gas-filled envelope. The lower electrode 4 of the deviceN is connected to a slider S adjustable along a further potentiometer Pthat is grounded at G1, for applying a bias potential, below theconduction threshold voltage of the tube N, to the electrode 4, in viewof the bi-lateral conduction properties thereof. The bias provided bythe slider S on the potentiometer P will be adjusted so that, uponturn-on of the voltage supplied to the vidicon 1, the target 3 can neverreach a potential sufficient to produce the substantial quantity ofsecondary electrons before referred to, since the neon device N willconduct before such potential is reached. The neon tube N, whichinitially presents an impedance very much greater than that of theseriesresistor R, will be caused to conduct if the target 3 starts toapproach too high a potential, thereby effectively connecting the target3 through the then low-impedance conducting threshold device N to theground terminal G1, and thence through ground to the ground terminal G2of the electron-gun cathode structure 5, Insurance is thus attained thatthe target 3, even upon turn on, will never reach that predeterminedvoltage at which the target would become an effective secondary emitter.The present invention therefore enables the use of all vidicons,irrespective of the peculiarities or variances in the characteristics oftheir target 3, with the type of light-compensating circuits beforedescribed.

It is important not only that the threshold-voltageoperated device Nhave an initial impedance, prior to conduction, that is high comparedwith the high-series resistance value R, required for automatic lightcompensation, but that the capacitance of the device N must besufficiently small that it does not disturb the high-frequencyperformance of the video amplifier output circuits connected to thecoupling capacitor C. It is fortunate that the threshold-voltage neontube before described has such a lowcapacitance of the order of a A of amicro-micro farad. Other types of two-electrode devices and the like,however, such as the ordinary type of semi-conductor, on

the other hand, not only do not provide a sufficiently higher resistancecompared to resistor R to effect the purposes of the present invention,but ordinarily they have far too high capacitance of the order of fromtwo to ten micro-micro farads, Similarly, thermionic diodes or devicesof this character also have too high a capacitance of the order of, say,four micro-micro farads, and, furthermore, have far too low a resistancein view of the heater-to-cathode leakage resistance of such diodes. Thepresent invention, accordingly, requires threshold-voltageoperateddevices that provide this very high initial resistance compared with theseries-resistance R and this very low value of capacitance.

Modifications will occur to those skilled in the art, and all such areconsidered to fall within the spirit and scope of the invention asdefined in the appended claims.

What is claimed is:

1. Apparatus for preventing secondary-emission effects from a vidicontarget and the like upon which electrons may be impinged from a cathode,having, in combination with the vidicon target, series resistance ofvalue comparable with that of the vidicon and connected between thetarget and a source of positive potential, and a circuit for preventingthe target from reaching a predetermined positive voltage value at whichthe target produces substantial secondary emission, the circuitcomprising a normally non-conductive threshold-voltage-operated deviceof impedance large compared with the said series resistance and meansfor biasing the device to a degree sufficient to insure that thethreshold conduction voltage thereof is produced for a value of targetvoltage below the said predetermined voltage value, the said circuitbeing connected between the target and the said vidicon cathode throughthe device when conductive.

2. Apparatus for preventing secondary-emission effects from a vidicontarget and the like upon which electrons may be impinged from a cathode,having, in combination with the vidicon target, series resistance ofvalue comparable with that of the vidicon and connected between thetarget and a source of positive potential, and a circuit for preventingthe target from reaching a predetermined positive voltage value at whichthe target produces substantial secondary emission, the circuitcomprising a normally non-conductive threshold-voltage-operated deviceof impedance large compared with the said series resistance and meansfor biasing the device to a degree sulficient to insure that thethreshold conduction voltage thereof is produced for a value of targetvoltage below the said predetermined voltage value, the resistance ofthe device, when conductive, being negligible, and the capacitance ofthe device being sufiiciently small to present negligible impedance tothe higher vidicon signal frequencies, and the said circuit beingconnected between the target and the said vidicon cathode through thedevice when conductive.

3. Apparatus as claimed in claim 2 and in which the said device is agaseous-discharge device.

4. Apparatus as claimed in claim 2 and in which the said device is atwo-electrode neon device and the like, one electrode of which isconnected to the target, and the other electrode, to the biasing means.

References Cited in the file of this patent UNITED STATES PATENTSShelton Mar. 24, 1960 Pike et a1 Dec. 20, 1960 OTHER REFERENCES

1. APPARATUS FOR PREVENTING SECONDARY-EMISSION EFFECTS FROM A VIDICONTARGET AND THE LIKE UPON WHICH ELECTRONS MAY BE IMPINGED FROM A CATHODE,HAVING, IN COMBINATION WITH THE VIDICON TARGET, SERIES RESISTANCE OFVALUE COMPARABLE WITH THAT OF THE VIDICON AND CONNECTED BETWEEN THETARGET AND A SOURCE OF POSITIVE POTENTIAL, AND A CIRCUIT FOR PREVENTINGTHE TARGET FROM REACHING A PREDETERMINED POSITIVE VOLTAGE VALUE AT WHICHTHE TARGET PRODUCES SUBSTANTIAL SECONDARY EMISSION, THE CIRCUITCOMPRISING A